The present invention relates to fiber optic transmission and, in particular, to the compensation for distortion created by such impairments as signal dispersion and nonlinearities.
It has long been recognized that the bit rate with which, and/or distance over which, information can be communicated over a fiber optic transmission channel is limited by various impairments introduced by the channel and/or the transmission equipment. These include, for example, polarization dispersion and chromatic dispersion. Specifically, as the bit rate and distance increase, the distortive effects of these impairments on the transmitted information also increase and, at some point, accurate information recovery becomes impossible. One impairment, chromatic dispersion, has a linear delay vs. frequency characteristic, which can easily be compensated for by simple linear equalization techniques when coherent detection (which preserves this linearity) is used. Moreover, one of us has found that the distortive effects of polarization dispersion can also be compensated for using simple equalization techniques if the degree of polarization dispersion is sufficiently small.
However, such equalization techniques may not be adequate. For example, direct, rather than coherent, detection is often preferred because it is less complex to implement. Unfortunately, though, chromatic dispersion gives rise to nonlinear distortion in the electrical signal at the receiver when direct detection is used. Such distortion cannot be compensated for using the aforementioned linear equalization techniques. Moreover, as polarization dispersion becomes larger, linear equalization becomes less practical and/or less effective because an increasingly large number of delay elements are required in the equalizer.
Adding to the foregoing problems are fiber nonlinearities which begin to manifest themselves as the capabilities of the channel are pushed to their limits through the use of increased signal power, higher bit rates, longer transmission distances and multiple channels. Firstly, these nonlinearities, by their very nature, introduce nonlinear distortion into the transmitted signals. Moreover, fiber nonlinearities, in combination with the chromatic and polarization dispersion, can increase the overall distortive effect to a degree which is greater than the sum of the individual effects.